If we had double track from Lowell Jct to the southern end of the Lawrence yard, a full length high level platform on the west track at both Ballardvale and Andover, and no platform on the eastern track at those two stations, a crossover for northbound commuter trains to move from the eastern track to the western track just south of Tewksbury St in Andover, a crossover facing the other way for freight trains just to the south of that, a crossover just to the north of both the Ballardvale platform and the Andover platform to allow a northbound commuter train to move from the platform to the eastern track, and a crossover just to the south of Essex St in Andover to allow a northbound train to get to the Andover platform from the eastern track, I suspect 15 minute rush hour headways with no freight is possible, with 30 minute mid day headways with freight.
It doesn't matter; they're not doing that. They have the MAB exemption in-hand for both stations to construct duplicate mini-highs on Track 2. That's what they're doing. No crossover games fed into a crapshoot as to whether everything's running on-time (because counting on Pan Am to run on-time is a fool's game). 2 tracks, 2 platforms, 2 mini-highs: that's the plan. They have the gapped full-high option to explore at some later date when they have the compatible cars for it and can perma-solve a more equitable solution.
I think the key to 15 minute headways would be to have each northbound commuter train scheduled to arrive at Ballardvale at the same time a southbound commuter train arrives at Andover. When a Downeaster is nearby, the Downeaster would use the eastern track, and whichever commuter train was going in the same direction as the Downeaster would use the eastern track to move between Ballardvale and Andover, with the commuter train going in the opposite direction of the Downeaster using the western track; when no Downeaster is around, having the peak direction train using the western track probably slightly improves comfort and speed for the predominant direction by avoiding the diverging moves through the crossovers.
Again...the Rail Vision is
NOT considering any 15-minute headways out in 495-land. Not even the kitchen-sink Alternative 6. Absolutely nowhere in the entire process and advocacy for RUR did anyone ever say :15 to the foot of the NH State Line was a thing we should be shooting for. The service proposal is for :30 frequencies on Lowell and Haverhill, interlined out to Wilmington for :15 coverage within Route 128...but 30-minute turns where the lines fork after that. The North Station throat can't handle 15 minutes to all five 495-land schedules to begin with, so right then and there it's moot. But also zero...nada...zilch calculated demand data the Rail Vision is based on points to a need for 15 minute turns that far outside of town. The last-mile feeders flat-out don't exist to sustain that kind of density. 30 minutes is a threshold that'll attract riders.
Twice that service level is a loss leader nobody can afford right now. I don't know where the idea came from that we need to crack the nut that gets 15-minute turns to all the hour-long schedules. Literally no one nowhere has ID'd that as a foreseable need.
Maybe in 30 years when car ownership has significantly changed in ways that are hard now to predict the RTA's will have filled out the suburbs with last-mile feeder buses. And then we can reach for some extra gear much more rapid than half-hourly, and have the NSRL + a maxed-out expansion North Station surface terminal ready to filet another massive frequency increase to absorb it all. But that is not a demand curve anyone can plot based on demographics in 2020. The trending that supports better-than half-hourly service does not exist.
15 minute headways with mostly single tracking is possible:
No, it's not. The Trillium Line isn't a mainline commuter RR with meets in mixed freight and intercity traffic 25 miles outside the CBD. It's a 5-mile, 5-station only light rail line inside city limits running DLRV vehicles on time separation from all other forms of line traffic, same as the NJ Transit RIVERLine. Nothing could be more unalike to the Purple Line or scaling therein. The
Reading Line is going to do :15 turns with all of Malden still being single-track, but the only ways it achieves that is by Haverhill thru service vacating to the Lowell Line, Reading Jct. in Somerville being modded for a 2 x 2 track split, Wellington passing siding being upgraded and lengthened, 2nd track being extended 1/4 mile from Ash St. Reading through the Reading platform, and North Wilmington being swapped over to the Wildcat with Haverhill schedules. Reading will be 2 segments of single-track 1 mile & 2 miles respectively...and 3 segments of double: 0.4 miles, 0.75 miles, and 6.3 miles. That's the minimum comfortable for managing a 12-mile Urban Rail schedule with 8 stations, conventional equipment, no other on-line traffic sources, and 1.8 miles of co-running with the Eastern Route in Somerville. And that is the
only stretch of single track on the system where they will be running 15-minute turns. All others are pre-existing DT mains, and the Old Colony is flat-out omitted from RUR except for some token off-peak backfills because it's not capable until somebody comes up with the $$$ for a Dorchester-Quincy ROW widening.
They aren't soliciting Innovation Proposals for traffic charts saying they're wrong, much less wrong because of an apples-turnips comparison from elsewhere. They've established what infrastructure thresholds they're comfortable running Urban Rail with chaos-proof OTP through, and below what thresholds they aren't. Just as they've established how many zones out :15 is warranted, how far out before :30 becomes the more effective target, and exactly why force-feeding :15 to the :30 folks is egregious waste they are decades of last-mile infill from being ready to support. The basic planks of the Rail Vision alts aren't up for total reboot. If they proceed with one of the max-build Alternative...
that's what it's going to be. Speculation about :15 minute turns out in 495 land is service-level Crazy Transit Pitches vs. what they are actually aiming to enact.